Colorectal cancer is one of the leading causes of cancer death worldwide, with the liver being the most common and critical site for development of colorectal cancer metastases. Surgical resection remains the only curative hope for patients with disease that is metastatic to the liver. However, 85% of these patients are ineligible for resection, with the 5-year survival for inoperative patients ranging from 0-5%. MicroRNAs (miRNAs) are non-coding RNAs approximately 20 nucleotides in length that exert their regulatory effects by binding to imperfect complementary sites predominantly located within the 3'UTR of their mRNA targets. Their causal role in promoting tumorigenesis, invasion and metastases has only recently come to light. Almost all cancer types show aberrant expression of miRNAs, with patterns of overexpression or, more commonly, underexpression in tumor cells. We hypothesize that these miRNAs constitute valid targets for the development of molecular therapies against metastatic disease. Here we propose to use the Sleeping Beauty transposon system to achieve extended and high level liver-directed expression of both tumor suppressor miRNAs as well as antimiRs against oncogenic miRNAs in the treatment of human colorectal cancer metastatic to the liver. Although it is well established that miRNAs can be transferred horizontally by a number of different mechanisms, in vivo data demonstrating exogenous miRNA transfer from normal cells to tumor cells in an animal model of cancer are lacking. In Aim 1, we have devised an approach to quantitatively evaluate intercellular transfer of miRNA from normal liver tissue into human tumor xenografts, assaying for miRNA delivery to an mRNA target by the effect on luciferase reporter gene expression in tumors. In Aim 2 we will test the effectiveness of tumor suppressive miRNAs (miR-34a, miR-9) as well as targeting an oncogenic miRNA (miR-21) both singly and in combination against human colorectal tumor xenografts in nude mice. Results from these studies will provide fundamental experimental support for therapeutic microRNA transfer from normal tissues into tumor cells in an in vivo model of metastatic disease. They will also provide preclinical support for an miRNA-based antitumor strategy against metastatic colorectal cancer that is potentially applicable to the treatment of an cancer that is metastatic to the liver.